The remaining service life of mechanical components that undergo repeated stress/strain is generally not readily predictable. Efforts have previously been made to predict the remaining service life of a component based upon the total time that the component has experienced stress/strain cycles. To ensure that a component is not used beyond its predicted lifespan, a component is often retired prematurely, that is, the component may be removed from service, often with significant remaining service life, just to be certain that the component will not fail while in use. As will be apparent, such premature removal of the component from service can be costly since at least a portion of the useful life of the component will be wasted.
By way of further explanation, reference is made to FIG. 1. In this figure, the narrower bell-shaped curve 10 on the right-hand side represents the probability density distribution, i.e., uncertainty, of component strength S, e.g., the distribution of stress/strain range values at a given fatigue life cycle value. The standard deviation, or half-width, of this distribution is determined by the amount of control resident in the component fabrication and constituent material refining processes. The broader curve 12 on the left hand side of FIG. 1 represents the overall spectrum of operational, or usage, stress/strain s that a component is expected to undergo during its operational life. This broader curve is derived from the total number of systems and the total set of conceivable operating conditions. Any single component is contained within a portion of the distribution at the right, and any single operational scenario is contained within a portion of the distribution at the left. Performance versatility dictates that the operational usage distribution 12 be as broad and inclusive as is tolerable. As used herein, “stress/strain” is to be interpreted as stress and/or strain.
In order to ensure that the component does not fail during its service life or at least to reduce the risk of the component failing during its service, the region in which the two distributions overlap, i.e., the region 14 that is shaded in FIG. 1, should be minimized or eliminated, if possible. In instances in which the two distributions overlap, an acceptable common point of operational stress/strain and component strength is employed as the assumed operating condition throughout the life of the component. This common point is at some value of operational stress/strain, s= s+qσs, wherein s is the mean operational stress/strain value, σs is the standard deviation and q is a predefined multiplicative factor. This value also has a corresponding value of component strength, S= S−nσs wherein S is the mean strength, σs is the strength distribution standard deviation and n is another predefined multiplicative factor.
This conventional approach suffers from at least two disadvantages. First, in instances of extreme exceedance of operational stress/strain which lie to the right of the strength curve 10, the impact of such extreme exceedance of operational stress/strain is not anticipated and requires human intervention to ensure that the component is appropriately monitored. Additionally, in the more common instance in which the operational stress/strain is lower than that defined by the assumed operating condition, the corresponding extension in the service life that such lower operational stress/strain provides is also not taken into account, thereby resulting in the premature retirement of components with unknown amounts of remaining useful service life.
A service life measurement system has been developed for continuously detecting and tracking operational stress/strain. Examples of a service life measurement system are described by: U.S. Pat. No. 6,618,654 to Stephen V. Zaat; U.S. patent application Ser. No. 11/473,418 filed Jun. 22, 2006 and entitled System and Method for Determining Fatigue Life Expenditure of a Component (hereinafter “the '418 application”); U.S. patent application Ser. No. 11/733,019, filed Apr. 9, 2007 and also entitled System and Method for Determining Fatigue Life Expenditure of a Component (hereinafter “the '019 application”), and U.S. patent application Ser. No. 11/697,661, filed Apr. 6, 2007 entitled Method and Apparatus for Evaluating a Time Varying Signal (hereinafter “the '661 application”), the contents of all of which are incorporated herein by reference. A service life measurement system is generally designed to continuously detect and track operational stress/strain, determine in real time or substantially real time stress/strain ranges that occur in critical areas of components during usage and calculate the duration of each stress/strain range cycle. Each operational stress/strain range that is determined represents a point on the broad operational stress/strain distribution curve 12 of FIG. 1. Of course, any measurement, be it direct or derived, has a mean, or measured, value M and an associated standard deviation, or uncertainty, σM, which is generally considerably less than σs. In order to be conservative, a measurement value of M+mσM can be assumed for each stress/strain cycle measurement, wherein m is a predefined multiplicative factor.
According to a service life measurement system, the corresponding total number of life cycles Nf that corresponds to the measured operational stress/strain range can be determined with the total number of life cycles Nf being those life cycles that the component could endure if it were to be operated solely at the stress/strain range that was measured. See, for example, the '418 and '019 applications which describe the determination of the total number of life cycles Nf associated with a particular stress/strain range. As further described by the '661 application, the time duration Δt associated with each stress/strain range cycle can be determined, such as through the application of rainflow sorting. Accordingly, a service life measurement system provides a technique for determining the stress/strain range cycles occurring at component critical areas along with the corresponding values for the total number of life cycles Nfi for each stress range Δσi (or strain range Δεi) in the time duration Δti of the stress/strain range cycle i.
A service life measurement system therefore provides a technique for more accurately determining the actual stress/strain that a component undergoes during usage and, in turn, the effect of such actual stress/strain upon the expected useful life of a component. Accordingly, owners or lessees of a component, particularly a component that is relatively costly and that has a relatively long lifespan, such as an aircraft, may be interested in the more accurate estimate of the remaining useful lifetime of the component so as to keep the component in service for a longer period of time and to avoid unnecessarily early retirement of the component as has been done in the past. In instances in which the more accurate estimate of the remaining useful lifetime of a component could be provided to the owner or lessee of the component, the owner or lessee of the component would likely be willing to provide compensation for the more accurate estimate, such as in terms of a percentage of the additional lifetime of the component that is provided by reliance upon the more accurate estimate provided by a service life measurement system or the like as opposed to that predicated upon the more conservative traditional approach of early retirement of the component. Accordingly, it would be advantageous to provide a measure of the additional useful life and its relative economic significance so that an appropriate valuation of the extension of the useful lifetime of the component could be determined.